Isobutene is an important industrial starting material that in recent years has been in the spotlight as a starting material for isobutene polymers and methyl methacrylate (MMA). In addition, isobutene commonly exists in C4 olefin mixtures that generally contain n-butene or the like, and it therefore becomes necessary to separate isobutene from these C4 olefin mixtures.
In one process for producing isobutene from C4 olefin mixtures, n-butene-free isobutene is produced by the combination of a hydration reaction and a dehydration reaction. In other words, isobutene in the C4 olefin mixture is selectively hydrated and separated by recovery as TBA and the resulting TBA is dehydrated to obtain isobutene. Moreover, the TBA yielded by hydration and recovery/separation is also an important industrial raw material. In particular, demand for TBA has been growing in recent years since it can be used directly without dehydration as a starting material for MMA.
The olefin mixture as a starting material is mainly raffinate 1 (commonly known as “spent BB”), which is obtained by the removal of butadiene from the C4 fraction from the naphtha cracking process and has isobutene and n-butene as its main components. Alternatively, the olefin mixture as the starting material is a butene-containing C4 fraction obtained from the FCC cracking of heavy oil. Isobutene and n-butene are present in both starting materials, but are difficult to separate by distillation since the difference between their boiling points is small at no more than 1° C.; reactive separation processes have thus been devised in which the isobutene is selectively hydrated.
A packed fixed bed reaction using a strongly acidic ion-exchange resin is one process for carrying out the hydration reaction (for example, refer to Patent Documents 1, 2 and 3). This hydration reaction process provides an excellent separation from the catalyst since the catalyst is fixed; a drawback to this process, however, is a low reactivity due to a low contact efficiency. In order to enhance the reactivity and hydration reaction selectivity, a process has been disclosed in which the reactivity is improved by the use of a suitable solvent, circulating the reaction solution, and using an optimal circulation range (for example, refer to Patent Document 4). On the other hand, a process is known in which, for example, benzenesulfonic acid, is used as the catalyst in a homogeneous hydration reaction (for example, refer to Patent Document 5). Processes are also known in which contact is effected with an aqueous heteropolyacid solution as a catalyst (for example, refer to Patent Documents 6, 7, 8 and 9). Problems requiring attention still remain with fixed bed reactions that use, e.g., an acidic ion-exchange resin, for example, as follows: a majority of the product exiting the reactor must be circulated, water is not to be mixed with the hydrocarbon that is the reaction starting material, and an organic acid and/or TBA (the reaction product) must be added as solvent in order to raise the wettability on the solid ion-exchange resin and thereby raise the contact efficiency. While the heteropolyacid process is an excellent process that provides a high reactivity and also a high selectivity for isobutene, the process as a whole is not entirely free of problems and some issues remain; improvements thereto have been carried out (refer, for example, to Patent Documents 10, 11 and 12).
However, for each of these production processes the problem still remains of being able to continue production on a long-term stable basis. Timewise changes occur, for example, as follows: although each catalyst exhibits an excellent initial activity, the catalytic activity undergoes a gentle but gradual decline over the timeframe of a year; the separation capacity of the catalyst separator declines; and the separation performance of the distillation column declines. One solution for this is to keep the continuous reaction running by discarding a portion of the catalyst that has undergone long-term use in the reaction and making this up with fresh catalyst. Within the context of the green chemistry movement seen in recent years, reducing the amount of this discard then becomes an issue.
Processes that are carried out in a homogeneous system using a strong acid, for example, sulfuric acid, have long been in general use for dehydration reactions. However, these processes are undesirable as industrial production processes because the high corrosivity due to the use of a strong acid requires a corrosion-resistant production apparatus and because it is difficult to treat the spent sulfuric acid discharged after the dehydration reaction. Several processes have been introduced in recent years that solve these problems by carrying out the dehydration reaction in a heterogeneous system using a sulfonic acid group-containing strongly acidic ion-exchange resin as the catalyst. In one process disclosed in this sphere, TBA is dehydrated at 80 to 150° C. on a fixed bed that uses an ion-exchange resin. However, when the reaction temperature in this process is 120° C. or below, due to the fact that the TBA and isobutene and water are in an equilibrium relationship, the post-reaction compositional proportion of the water rises and the reaction rate declines. This results in a low one-pass conversion, making it necessary to recycle large amounts of TBA and water and causing a complex production process. When the reaction is carried out at temperatures above 120° C., isobutene oligomerization, a secondary reaction, proceeds and the yield is reduced (for example, refer to Patent Document 13).
On the other hand, numerous processes are also known for the gas-phase production of isobutene using a solid acid catalyst. Since the dehydration reaction of alcohols is a highly endothermic reaction, it is generally carried out at high temperatures, i.e., 250° C. and above, in the gas phase, as is done in Patent Document 14. A characteristic feature of this dehydration reaction is the production of, for example, diisobutene, by the oligomerization of the isobutene product, and the reaction is preferably run at higher temperatures in order to inhibit this oligomerization. However, raising the temperature results in an acceleration of carbon deposition on the surface of the catalyst, causing a timewise decline in the activity of the catalyst and having a negative influence on long-term stable industrial production.
In the production of olefin by alcohol dehydration, the development of a catalyst that can maintain its catalytic activity long term is very significant for gas-phase reactions that use a solid acid catalyst, and several processes have been quite recently proposed. One such process that has been proposed is the use of γ-alumina as the catalyst for producing lower olefin by the dehydration of C2 to C4 lower alcohol (for example, refer to Patent Document 15). However, according to the disclosures in Patent Document 15, isobutanol is used as the starting material for the isobutene production processes in the examples, and what the results would be for the use of TBA as the starting material cannot be predicted. In addition, high pressures are required in order to achieve the object and the isobutene yield in the examples is also not satisfactory.
Thus, as described above, problems still remain with the heretofore known processes, both with the processes carried out at relatively low temperatures in the liquid phase and the processes carried out at high temperatures in the gas phase. With regard to processes using a solid acid catalyst in the gas phase, the current situation, in accordance with the preceding description in connection with Patent Document 15, is that there has been no report of a catalyst capable of long-term maintenance of its catalytic activity in the production of isobutene from TBA in the gas phase using a solid acid catalyst.    [Patent Document 1] Japanese Patent Application Laid-open No. S56-10124    [Patent Document 2] Japanese Patent Publication No. S56-2855    [Patent Document 3] Japanese Patent Application Laid-open No. S55-64534    [Patent Document 4] Japanese Patent Application Laid-open No. H11-193255    [Patent Document 5] Japanese Patent Application Laid-open No. S55-51028    [Patent Document 6] Japanese Patent Application Laid-open No. S54-160309    [Patent Document 7] Japanese Patent Application Laid-open No. S55-7213    [Patent Document 8] Japanese Patent Application Laid-open No. S55-62031    [Patent Document 9] Japanese Patent Publication No. S58-39806    [Patent Document 10] Japanese Patent Application Laid-open No. 2000-44497    [Patent Document 11] Japanese Patent Application Laid-open No. 2000-43242    [Patent Document 12] Japanese Patent Application Laid-open No. 2000-44502    [Patent Document 13] Japanese Patent Application Laid-open No. S58-116427    [Patent Document 14] Japanese Patent Publication No. S48-10121    [Patent Document 15] Japanese Patent Application Laid-open No. H4-300840